I bought an already built house on Sims 3 but now I want to build a second floor but when I try to put stairs it says ceiling to low. How do you make the roof higher?

When your in the build mode, just click on the floor boutton in the bottom left with the up arrow.

You have to go to the build section and buy stairs and then you have to make a floor.

go to the build mode and when you do the base of the house, there should be a small picture of stairs, if you click on that, it will bring up the stairs.

Step 1: Go to Build mode and go to walls, and click and drag the highlighter and make the outside shape of your house. Then, when you've decided and made your outside walls, make room divider walls,...

I think if you want a second floor you can buy it in the shop thing on there or you can buy another house with a second floor.